Hi FedUp74,
Welcome and hello

It says a lot about your strength that you can depersonalize the insults. That takes a lot of emotional maturity!
I’m struggling with how to do that without agreeing with his position (which is usually that I did something wrong or I’m ignorant, etc... ). For example, my BF often tells me that I humiliate him in public – I’ll say something that triggers a negative reaction from him and he’ll accuse me of doing it on purpose. I typically apologize and explain that it wasn’t my intent to humiliate him (although I don’t think a non BPD would find it humiliating). I now feel like I have to be hyper aware of everything I’m saying to make sure it can’t be perceived the wrong way – sometimes I slip up which triggers his reaction. How do I counter his perception that I’m doing it on purpose?
Apologizing is a little different than validation.
Validation is about recognizing that he has that feeling -- he feels humiliated. No point in arguing with that.
"You felt humiliated when you thought I ignored you in public. I can understand why someone would feel that way."
When his emotions are swelling, he's in emotion mind. You are in rational mind. Anything that gets said that doesn't confirm how he feels in emotion mind will be discarded, anyway. When you use validation, you're speaking to his emotional state. "Yes, I understand you feel humiliated." And mean it

He felt something happened (that didn't), and that made him feel humiliated. Usually, if you can depersonalize a little and have empathy for this scenario he feels actually happened, you can see how he might feel that way.
This is different than agreeing that you humiliated him.
If you defend yourself (JADE = justify, argue, defend, explain), you are using a language he is not able to speak at the moment. It just pisses him off and gives him a new thing to be angry about.
If it's something important where the truth has to be stated, it's often better to wait until he has returned to baseline.
Also, in my experience validation tends to work best before a full-blown dysregulation. After dysregulation, starts, you might have to go straight to boundaries if the dysregulation is cresting and about to engulf everyone and anything in sight. "I won't be yelled at so I'm going to exit stage left here. I want to hear what you are saying, and to do that, I need to wait until my nerves have calmed down. I'll be back in 2 hours."
I have found
validating questions to be a really helpful way to ease into this.
Oh?
How did you feel about that?
What did you do?
And then what did you do?
What would you like to do?
When do you think it could be done?
What do you think the outcome will be?
What do you think might work?
What do you think would work next time?
Are there other options?
What happened?
How did it happen?
Where did it happen?